Field of the Invention
The present invention relates to a method and system for controlling continuously variable valve timing (hereafter referred to as a “CVVT” for convenience), and more particularly, to a method and system for controlling CVVT that may prevent a decrease of engine speed due to retarding (or a retard angle) of the CVVT according to a negative cam torque by estimating directivity of cam torque when the CVVT first operates.
Description of Related Art
In general, internal combustion engines, which are apparatuses that generate power by receiving air and fuel from the outside and burning them in a combustion chamber, include an intake valve for drawing the air and fuel into the combustion chamber and an exhaust valve for discharging explosion gas burned in the combustion chamber. The intake valve and the exhaust valve are opened or closed by rotation of a camshaft that rotates with rotation of a crankshaft.
It is required to make timing of opening/closing the valves different in accordance with engine speed, engine load, and the like depending on traveling conditions of a vehicle, in order to increase efficiency of the engine.
In particular, the timing of opening/closing the intake valve has a large influence on efficiency of an engine. That is, when the intake valve is opened in advance, since an overlap period of the valves increases and intake/exhaust inertia flow can be sufficiently used at a high speed, the efficiency of the engine increases, but at a low speed, the efficiency decreases since the amount of remaining gas increases, thus discharge amount of hydrocarbon (HC) increases.
Therefore, a technology that does not set an overlap period of the valves of a camshaft in accordance with the rotation of the camshaft to appropriately control valve timing in accordance with a driving state of the engine has been developed and applied, which is referred to as a CVVT.
The CVVT is an apparatus (or system) that continuously changes an opening time of the intake valve or the exhaust valve by changing a phase of the camshaft at the intake side or the exhaust side in accordance with load of a vehicle, engine speed, and the like. In other words, the CVVT changes the valve overlap period. An object of the CVVT is to reduce exhaust gas, to improve performance, and to stabilize idling.
The valve timing means a period that the intake valve or the exhaust valve opens or closes, and an intake process is a process from opening to closing of the intake valve, while an exhaust process is a process of discharging exhaust gas for a period from opening to closing of the exhaust valve. The timing of opening or closing the valves influences performance of the engine.
The valve overlap means a period that the intake valve and the exhaust valve are simultaneously open. Once the period of the valve overlap is set in a typical engine, it is constantly used in all engine speeds, such that it is disadvantageous at a low engine speed or a high engine speed.
Accordingly, controlling of the valve overlap to fit the engine load consequently improves the engine output, such that an apparatus therefor is a CVVT apparatus or system.
The CVVT apparatus includes a continuously variable valve timing unit, an oil control valve (OCV) that is an oil supply device, an oil temperature sensor (OTS), an oil control valve filter, an oil path, an auto tensioner, etc.
The continuously variable valve timing unit may be mounted on, for example, the camshaft at the exhaust side, the inside thereof includes a housing and a rotor, an advancing (advance angle) chamber and a retarding (retard angle) chamber are provided between the housing and the rotor vane, and oil flows inside through the OCV, such that a rotor vane moves.
Further, the OCV is a core device of the CVVT apparatus, and controls the valve opening/closing time by changing a path of engine oil which is supplied from an oil pump and flows in the continuously variable valve timing unit according to controlling of an engine electronic control unit (ECU).
Density of the engine oil that is a working fluid of the continuously variable valve timing unit changes in accordance with temperature. The OTS measures temperature of the engine oil to compensate for a changed amount of the engine oil density due to the temperature before the engine oil flows into the OCV, and sends the measured temperature to the ECU. The ECU compensates for the changed amount of the engine oil density by driving the OCV based on the measured temperature.
Further, the oil valve control filter filters impurities in the engine oil flowing to the OCV, and the auto tensioner, which is a tension control device of a chain connecting a sprocket of a camshaft at the exhaust side where the CVVT apparatus is disposed with a sprocket of the camshaft at the intake side, ensures stability of performance by preventing delay or deviation of responsiveness and problems in function of the CVVT apparatus.
When the CVVT apparatus controls a position of a cam, not in a maximum retarding angle at the intake side or in a maximum advancing angle at the exhaust side, but therebetween, this control is referred to as intermediate phase CVVT control. According to the intermediate phase CVVT control, operating responsiveness of the CVVT apparatus is good, and a usage range of the cam may widen, thus fuel efficiency is improved and reduction of exhaust gas is achieved.
As shown in FIG. 1, when the ECU 1 applies a pulse width modulation (PWM) signal to the OCV 10, a plunger 12 of the OCV 10 moves toward a target position, such that a position of the cam 20 of the CVVT apparatus 100 is controlled toward a parking position, a retarding position, a holding position, or a advancing position depending on an oil flow path 14. In other words, the oil flow path 14 is changed depending on a position of the plunger 12, thereby operating the CVVT apparatus 20.
The position of the plunger 12 is controlled depending on a duty value of the PWM signal. As shown in FIG. 2, since the position of the plunger 12 is changed in order of the parking position (or a detent position, a locking position), the retarding position, the holding position, and the advancing position, the position of the cam of the CVVT apparatus is controlled depending on the position of the plunger.
In the intermediate phase CVVT apparatus, when the engine is started, the cam is positioned at the parking position, and when an operating condition of the CVVT is satisfied, the ECU 1 controls a position of the cam. When the ECU controls a position of the cam from the parking position to the advancing position or the holding position, the oil flow is changed to an advance oil flow or a holding oil flow passing by a retard oil flow. In this case, when oil flows in the retard oil flow path under adverse conditions, since the CVVT apparatus may be momentarily retarded, the engine speed may decreases, starting stability may deteriorate, and customer complaints may occur.
Meanwhile, as shown in FIG. 3, a cam torque occurs depending on movement of the cam 20 in an engine system for a vehicle. The cam torque may periodically have a positive value or a negative value. The cam torque is affected by a cam, a chain device, a high pressure pump and so on which are provided in the CVVT apparatus.
The positive cam torque acts in an advancing direction of the CVVT apparatus, and the negative cam torque acts to a retarding direction thereof.
Accordingly, while the CVVT apparatus operates, it is required to control an operating condition of the CVVT through determining whether the cam torque is positive or negative using a crank sensor and a cam sensor.
In the intermediate phase CVVT apparatus, when the engine is started, the cam is positioned at the parking position. After the engine is started, when the intermediate phase CVVT apparatus first operates and the cam is controlled and changed from the parking position to the advancing position or the holding position and at this time the retard oil flow path serves, the CVVT may be momentarily retarded, thus engine speed may decreases. In this case, when the cam torque is negative, since the cam torque is applied to the cam in the retarding direction as an adverse condition, problems with respect to undershoot of the cam and decrease of engine speed may deteriorate.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.